The continuous casting system provides a system of casting fabrication in which a supply of molten metal or metal alloy is heated and liquified within a furnace-like structure called a tundish or heated outside the tundish and placed therein prior to casting. In most systems, the furnace or tundish includes a discharge orifice near the bottom of its internal cavity which is coupled by a passage to a cooled die or mold. The latter defines an entrance opening and an exit opening and an elongated die passage suitable for the formation of an elongated casting. In addition, cooling means are provided which generally encircle or surround the die passage for the purpose of conducting sufficient heat from the molten metal within the die passage to solidify all or part of the molten metal therein and form the casting. Continuous casting systems may comprise either vertical or horizontal casters.
Vertical casting systems are generally used to form large plate-like castings and acquire their name from the vertical path which the casting travels. The furnace and cooled mold are arranged vertically and gravity flows the molten metal from the furnace into and through the mold. In most vertical casting systems, an array of drive rollers beneath the mold control the downward progression of the casting. In addition, many vertical casting systems include means which introduce a gradual curve into the casting to transition it from a vertical path to a horizontal path in order to reduce the overall height of the casting system.
The horizontal continuous casting process acquires its name from the horizontal arrangement of the tundish and cooled mold as well as the horizontal path which the casting travels. This provides a system in which a supply of molten metal is caused to flow horizontally to a cooled die which defines a horizontal elongated die passage in which an elongated casting is formed. The cross section of the cooled die passage determines the cross section of the continuously formed casting. Coolant is circulated through the cooled die to carry heat from the introduced molten metal with sufficient speed to cause the molten metal to solidify or freeze on at least its outer surfaces, that is the surfaces proximate to the die passage surfaces. A solidified peripheral region of the casting is thus formed which "grows" inwardly toward the core of the casting within the die passage as heat continues to be carried from the metal.
Generally, the length and capacity of the cooled die and speed of casting motion are selected to ensure that the casting emerging from the die passage is at least solidified about its outer surface and inward therefrom to a depth sufficient to permit the casting to be pulled by a pair of motor driven rollers downstream of the die passage. While the continuous casting process is generally described as a process in which the molten metal within the furnace is "continuously" flowed into the die passage from the casting, the actual casting motion is more "step-like" than continuous. That is to say the casting is periodically moved forward in a series of short steps of a predetermined distance called "strokes". Between strokes the casting is stopped for a brief time or in some systems a brief backspace of the casting occurs between forward strokes to reduce boundaries within the material. Thus the process is, in one sense, a misnomer in that the motion of the casting is not truly continuous, but rather, is periodically stopped and in some instances, reversed, for a brief "backstep". However, the process continues to be known in the art as continuous casting because it provides an emerging casting having virtually unlimited length and a cross sectional shape determined by and conforming to that of the die passage.
In both horizontal and vertical casting systems, the formation of thicker casting configurations results in withdrawing the casting from the cooled mold before complete solidification has taken place. As a result, the casting emerging from the cooled die passage has the above-described solidified outer skin and molten center. The heat present in the molten casting center can cause the casting skin of the casting emerging from the cooled die passage to be melted. Practitioners in the art have attempted to meet this problem, known as "remelting", by utilizing a cooled die or mold which is long enough to have sufficient cooling capacity to withdraw substantially more heat from the casting than the minimum required to form the above-described skin. Often such long molds have die passages with lengths equal to seven to ten casting strokes. The use of long casting molds provides some additional cooling of the casting. However, the effectiveness of long molds in the continuous casting process is limited due to the shrinkage which the casting undergoes as cooling takes place. This shrinkage tends to distribute itself down the casting and result in a reduced cross-sectional area and surface area of the casting as a function of distance from the tundish. In essence, the casting assumes a "tapered shape". In most castings, the casting taper is sufficient to cause an air space to be created between the casting skin and the surfaces of the cooled die passage as the casting "shrinks" away from the passage walls. Once the contact between the passage walls and the casting surface is broken, the cooling of that area of the casting is decreased reducing overall cooling and creating "hot spots" in the casting. It is the creation of these hot spots which may produce a localized remelting of the solidified casting causing molten metal from the casting interior to flow out beyond the casting skin and damaging the casting. Even if actual remelting is avoided, the portions of the casting remaining in contact with the die passage are cooled more rapidly than those no longer in contact. As a result, uneven cooling takes place which degrades casting quality and may actually cause the casting to warp or bend.
The problems associated with casting taper in long molds have prompted practitioners in the art to attempt to compensate for casting shrinkage by simply constructing the die passage of the long mold to include a carefully designed taper which gradually narrows the die passage as a function of distance from the entrance orifice or tundish.
The use of tapered die passages within the mold structures provides some improvement in the ability of the cooled die to compensate for the shrinkage of the casting. However, the determination of appropriate casting taper is complex and the use of a tapered mold involves careful control of the system operation. For example, each casting configuration and size and each metal or metal alloy used requires a different shrinkage taper. In addition, for each casting and metal or metal alloy cast, the passage taper is fitted to a casting stroke and speed. Therefore, the casting stroke and speed must be inordinately controlled. Simply stated, the mold or cooled die taper must be customized for each application. This leads to increased fabrication and tooling costs which are prohibitive in a competitive environment. Further, tapered molds or dies are less tolerant of wear due to the precision required of the taper. Such systems, as shown and described in U.S. Pat. No. 3,580,327, U.S. Pat. No. 4,308,774, and U.S. Pat. No. 3,467,168, provide structures which contact only portions of the casting surface. As is well understood by those skilled in the casting art, complete contact with the entire casting surface including its corners is essential to the attainment of even cooling of the entire casting in order to provide the desired casting uniformity and grain structure as well as prevention of the remelt phenomenon.
While the above-described prior art long mold casting structures have provided some improvement in casting cooling and a partial solution to the problem of accommodating casting tapers, they are subject to the above-described problems of accommodating casting taper. There remains therefore, a need in the art for an improved cooled mold for use in continuous casting systems which effectively withdraws heat from the emerging casting and which avoids the need to utilize a custom tapered long mold structure.